The field of the invention is power steering apparatus for vehicles, especially those whose steered wheels are turned by one or more hydraulic actuators, and that are primarily for off-highway applications such as agriculture or earth moving. Many prior power steering systems for use in such vehicles have hydraulic hoses and/or mechanical linkages entering into the cab of the vehicle. In another type, which can be called a steer-by-wire system, only electrical cables are permitted in the driver's cab.
For steer-by-wire systems, the following three approaches among others are possible: (a) open-loop control of steered wheel motion based upon the velocity of the steering wheel; (b) direct position control of the steered wheels from the steering wheel; and (c) torque demand control, wherein the torque at the steered wheels is a more powerful copy of the torque applied to the steering wheel by the the operator. The torque demand control systems, type (c), have the advantage over (a) and (b) of feeling closest to the way familiar power steering systems feel to the driver.
One steer-by-wire system of type (c), disclosed in a co-pending application of the same assignee as this invention, utilizes an electric motor in the cab to provide reaction torque to the steering wheel and an electromechanical torque sensor in the cab to produce torque demand signals for steering.